This is the way I teach to solve eaquations like
sinx=0.4
x=sin-10.4=23.58
Then the other solutions are
180-23.58, 360+23.58, 540-23.58, 720+23.58, 900-23.58,1080+23.58 etc
See the pattern?
cosx=0.4
x=cos-10.4=66.42
Then the other answers are
306-66.42, 360+66.42, 720-66.42, 720+66.42, 1080-66.42, 1080+66.42 etc
Again the pattern
To solve tanx=0.4
x=tan-10.4=21.80
then the other answers are180+21.8, 360+21.8, 540+21.8, 720+21.8, 900+21.8 etc